Ductwork is typically employed in combination with air-handling units (heaters, air-conditioners, filters, humidifiers, de-humidifiers, etc.) for conveying and distributing air from the air-handling unit to a room or other enclosure. Ducts are typically formed of a rigid metal material such as steel, aluminum, or stainless steel. In many environments, ducts are hidden above suspended ceilings. In warehouses, manufacturing plants and other similar buildings, however, the ducts are suspended from the roof of the building and are thus exposed. Metal ducts may be problematic in those warehouse or manufacturing environments where prevention of air-borne contamination of the inventory is critical. For example, temperature variations in the enclosure, or temperature differentials between the ducts and the air being conveyed can lead to condensation of moisture on the ducts. The presence of such moisture may in turn lead to the formation of mold or bacteria on the duct, which is then distributed through the enclosure by the air passing through the ducts. Further, in the case of exposed ducts, condensation on the exterior of the duct can result in such condensation dripping from the duct onto the inventory or personnel below. Further still, ducts often have a layer of insulation bounded to the exterior by a further layer of aluminum. The insulation layer ages and deteriorates as the duct cycles through natural temperature-induced size variations. As a result, the deteriorated insulation material may fall from the ducts.
Indeed, given this potential for contamination, the type of metal ducts that can be used in certain facilities may be subject to government regulation, which typically adds to the cost of using metal ducts. Food-processing facilities, under USDA regulation, have been required to use expensive, non-toxic sealed and insulated stainless steel ducts. As an alternative, such facilities could not use ducts at all. This may also be problematic as the air that cycles through the air-handling unit is not conveyed and distributed to the entire enclosure, but rather only cycles in the vicinity of the air-handling unit.
Metal ducts may also be problematic in that they promote drafts, creating localized temperature variations within a facility. As is well known, metal ducts include spaced diffusers to allow the air flowing through the duct to be distributed into the room in which the duct is located. Even so, the position of the duct and the flow rate therethrough can lead to undesirable local temperature variations. For example, the required controlled temperature in certain food-processing facilities is 42 degrees F. A food-handling worker standing just beneath a vent or diffuser in such an installation may have a draft of the cooled air blowing directly on him, while the worker next to him will not be subjected to such a draft. Such drafts can lead to lower productivity or even health problems for prolonged exposure.
A solution to both the expense of certain metal ducts and to the problem of drafts and uneven temperature distribution may be found in fabric air dispersion devices. Fabric air dispersion devices, such as that sold under the name DUCTSOX by Frommelt Safety Products of Dubuque, Iowa, are simply soft-sided tubes made of a fabric which may be air-porous. Such air-porous fabric air dispersion devices typically do not include openings or vents as in conventional ducts. Rather, the fabric air dispersion devices contain the forced air and inflate under the velocity pressure and static pressure from the air-handling unit to which they are attached. Given that the fabric is air-porous, however, the contained forced air slowly leaks out of the fabric air dispersion device along its entire length. Such a device thus provides an even temperature distribution without drafts. Moreover, such fabric air dispersion devices are easy to manufacture, inexpensive to ship (because of reduced weight and damage) and install, and are typically much less expensive than their metal duct counterparts. Such fabric air dispersion devices have been approved by the USDA for certain food-handling installations.
A potential disadvantage of fabric air dispersion devices, however, is their tendency to accumulate moisture in high humidity situations. For example, in certain food-handling facilities, a high-pressure, high-temperature steam washdown of the facility walls, equipment, floors, etc. must be performed daily. This significantly raises the humidity in the room. Although, by nature, the air-handling unit may be equipped to reduce such humidity, the air must be cycled through it repeatedly to gradually lower the humidity. Accordingly, high humidity air passes through the fabric air dispersion device. The humid air flow through the pores of the fabric may result in the accumulation of water condensation on the fabric. This is not only undesirable from the standpoint of potentially impairing the function of the device, but also because it reduces the rate of humidity reduction. A potential solution to this situation is to remove the fabric air dispersion devices during the wash-down operation. Given the simplicity of device installation, this could be done without significant effort. However, the absence of a means for conveying and distributing the air from the air-handling unit inhibits circulation of the air from the air-handling unit and through the entire room or enclosure, which circulation is helpful in uniformly reducing the humidity level in the room. Rather, the result is a significant reduction in humidity adjacent the air handling unit, or units, but continued high humidity elsewhere.